Trapped in Dope Sheet view
Ah! I tried ctrl-spacebar, which had no effect, but then accidentally tapped the alt key (yes, clumsy of me, I was trying to do this while holding a cup of tea), and the original view came back. 

So the answer is, 3-key combination: Ctrl - Alt - Space bar.
This toggles 'full screen' for whatever view the mouse is hovering over.

Thanks moonbots!